Pedogenic Factors Affecting Magnetic Susceptibility of Northern California Soils

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Abstract
Mass magnetic susceptibility (χ) was measured on samples taken from horizons from 27 northern California soils to determine the amount and vertical distribution of χ, and if the observed χ distribution could be related to parent material, climate, topography and time. In most of the soils with vertical morphological distinctions, eluvial horizon χ was greater than illuvial, R or CR χ. This susceptibility enhancement was interpreted as being due to pedogenic processes. Soils formed on parent material rich in Fe‐bearing minerals (e.g. basalt) had higher absolute χ and greater χ enhancement than did soils formed on Fe‐poor parent material (e.g. siltstone). Enhancement was greater in soils from areas of high (>1000 mm) mean annual precipitation compared with areas of low (<500 mm) mean annual precipitation, and soils formed under mean annual temperature ≤6 °C had less distinct enhancement than soils formed in warmer temperature regimes provided the precipitation was the same. Two soils with poor and somewhat‐poor drainage classes had distinctly lower absolute χ than associated well‐drained soils. Both absolute χ in eluvial horizons and susceptibility enhancement of eluvial over illuvial, and eluvial over C horizons increased as soils became older. Older soils were enhanced to a greater depth than younger soils. Eluvial and illuvial χ was greater than parent material χ in most profiles because non‐ or weakly magnetic minerals were converted to maghemite, which accumulated in eluvial horizons. In young soils with little morphological differentiation formed on transported parent material, χ measurements help to locate lithologic discontinuities.

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